Indexing drives for imparting intermittent motion have existed in the prior art. One such classic device is known as the Geneva mechanism. With the Geneva mechanism, constant rotation of an input shaft is converted to intermittent angular rotation of an output shaft which is parallel to the input shaft. The Geneva mechanism typically has a single pin, carried by the input shaft, which moves into and out of a series of straight radial slots in the periphery of a disk on the output shaft. The pin engages one of the slots in the disk as the input shaft rotates, accelerating and then decelerating the output shaft from and back to a rest position while rotating it through an angle of, for example, 120 degrees or less.
The Geneva mechanism has many limitations. The limitations are discussed, for example, in U.S. Pat. No. 3,443,455, which is directed toward an improvement in intermittent motion devices. A further improvement is disclosed in U.S. Pat. No. 3,835,723. These improvement devices employ a pair of pins and pairs of corresponding slots. One of the pins and one slot of each pair cooperates to accelerate the output shaft from rest to a maximum angular velocity and the other one of the pins and the other slot of each pair cooperates to decelerate the output shaft back to rest or zero angular velocity. These improved devices sought to overcome some of the disadvantages of the earlier Geneva and other types of indexing mechanisms, particularly the abrupt changes in acceleration of the output shafts such as occur in the Geneva mechanism upon engagement and disengagement of a radial slot on the output shaft by the pin on the input shaft. In the devices of the above-identified U.S. patents, the pair of pins on the driving shaft engage a corresponding pair of slots on the driven or output shaft, each of which has an opposite curve that determines the output shaft's motion, thus providing greater flexibility than with a single slot of the conventional Geneva mechanism by allowing shaping of the slots. The devices of the above-mentioned patents employ circular slots.
Providing an improved indexing mechanism in a right angle drive has presented greater difficulty. One such device is disclosed in U.S. Pat. No. 3,583,258. That device seeks to solve one of the problems found in prior mechanisms by eliminating the abrupt acceleration from, and deceleration to, zero angular velocity by providing a hemispherical slotted output member in which a pin on the input or driving element engages the slot of the output or driven element through a full 180 degrees. In the device described in U.S. Pat. No. 3,583,258, the pin enters the slot tangent to the slot direction in order to provide smooth beginning and ending acceleration at the rest position. With such a device, the hemispherical surface terminates in a planar edge which lies in a diameter plane through the center of the hemisphere, the plane being at right angles to the output axis of rotation and containing the input axis of rotation. The motion of the output element with such a device is described therein as otherwise corresponding to that of a traditional Geneva mechanism.
The traditional Geneva mechanism produces an output motion which has two additional undesirable features. The Geneva mechanism has a relatively high maximum acceleration and maximum deceleration, and additionally produces a rapid transition from acceleration to deceleration at the maximum angular velocity point. This imparts a rapid acceleration change rate, or jerk, to the output shaft. The jerk is in the form of a sudden torque reversal. This may be represented by an equation of motion in which the third derivative of the angle of the output shaft, or the rate of change of the angular acceleration of the output shaft, is not zero, but rather is a high negative number at the point of maximum output angular velocity. Such motion characteristics impart significant stress to the output drive train and to objects being handled, processed or moved by the driven apparatus, as well as wear to the mechanism itself.
Devices of the prior art have not provided the non-planar indexing mechanism, that is one in which the input and output shafts are not parallel, which is capable of overcoming high acceleration, and overcoming a high rate of change of acceleration, particularly at maximum velocity and at transition points in the intermittent drive cycle where mechanical components make and break contact. The prior art has further failed to provide an indexing mechanism which has acceptable characteristics which are generally adaptable to devices in which a driving and a driven shaft intersect at an angle that may be other than 90 degrees.